In intensity modulators for modulating intensity of an AC lamp based on pulse width modulating control, a light switching element is arranged in front of the AC lamp. Output light from the AC lamp enters the light switching element arranged in front of it, and its intensity is modulated by the light switching element. In this case, a driving current of the AC lamp synchronizes with a vertical synchronizing signal so that its polarity is inverted, and the driving current becomes zero at the inverting timing. The intensity of the output light from the AC lamp fluctuates with respect to the zero point of the driving current, and intensity error or flicker occurs in the light whose intensity is modulated by the light switching element. In the prior examples, therefore, during a period where the output light from the AC lamp fluctuates, the light switching element is controlled to be off so that the fluctuation of the output light is suppressed, but such suppression reduces the intensity of the output light according to a period where the light switching element is controlled to be off.